1. Field of the Invention
The present invention generally relates to a control device for an engine. More specifically, the present invention relates to technology for preventing surging in the turbocharger compressor of an engine equipped with a turbocharger and an EGR device.
2. Background Information
The idea of supercharging an engine using a turbo-supercharging device (turbocharger) comprising a turbine installed in an exhaust passage and driven by exhaust gas and a compressor installed in an air intake passage and driven by the turbine is well-known. When a turbocharger is used, there is a minimum allowable compressor flow rate that depends on the rotational speed of the turbine or the supercharging pressure. If the turbocharger is run with a compressor flow rate that is below the minimum allowable flow rate, a backflow of intake air moving in the upstream direction from downstream of the compressor will occur intermittently. This intermittent backflow is called “surging.” Surging is known to occur when the fuel injection quantity decreases and the exhaust pressure declines. A conventional method of avoiding surging is to provide a bypass that links the upstream side of the compressor to the downstream side and arrange for the bypass to open when the fuel injection quantity decreases, thereby securing a compressor flow rate at least as large as the minimum allowable flow rate. One example of such a control method is disclosed in Japanese Laid-Open Patent Publication No. 06-069331 (See, paragraph 0005).
The idea of supercharging an engine by providing an intake air throttle valve in addition to a turbocharger is also well known. The intake air throttle valve can be driven independently of the accelerator depression amount and is configured to reduce the opening area of the air intake passage when it is driven in the close direction. In this kind of engine, when the fuel injection quantity decreases, the supercharging pressure is reduced by driving the intake air throttle valve in the close direction. One example of such a control method is disclosed in Japanese Laid-Open Patent Publication No. 2000-170588 (See, paragraph 0012).
Engines that are provided with both a turbocharger and an exhaust gas recirculation (hereinafter called “EGR”) device for recirculating a portion of the exhaust gas to the air intake passage are also well known. The EGR device generally comprises an EGR passage linking the exhaust passage and air intake passage together and an EGR valve that is provided in the EGR passage and configured to control the flow rate of the recirculated exhaust gas.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved engine control device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.